Roofs and walls of a building or structure must deal with various environmental and interior building factors such as heat insulation, proper venting and moisture control. The design of the roof and wall systems needs to be structurally sound while providing for a habitable temperature living space. There are three sources of heat that affect the ability of a building structure's insulation to control heat transfer. The “insulation” or heat control of the temperature controlled space may be broadened to include dealing with moisture.
Convective heat transfer is the movement of air either naturally as in where hot air rises displacing the cold air (moves the cold air down), or the mechanical convection when air is physically moved such as by a fan. Convective heat movement accounts for about 15 to 20% of heat loss/transfer (air leakage) through exterior walls.
Conductive heat is the transfer of heat energy from one object by touch to another object. Thermal bridging of solid studs in a wall is an example of conductive heat loss bridging between external face covering to interior wall covering.
Radiant heat is the absorption of radiant energy emitted by a heated object. Approximately 60 to 75% of total heat transfer in a residence or building occurs through radiation, can be as much as 90% heat gain in the attic/ceiling in the summer time.
Improper venting and moisture control can impact the ability of the wall or roof system to control heat (insulation saturation with moisture and collapse) and to maintain the structural integrity of the building (rot, decay, or insects). Moisture control and drainage in the exterior wall is critical in certain applications such as brick and stucco where there may be through face leakage.
Heat insulation material placed in/or part of walls, ceiling, roofs, floors and other “wall structures” have historically been comprised of fibrous blanket insulation, such as fiberglass or foam insulation board. The principle of the blanket insulation is to form dead air spaces that provide insulation against convection and conduction heat transfer. The blanket insulation can be formed in small “clumps” and blown into spaces such as into the attics of residential homes and other building structures, and can also be made into elongated blankets formed in a specific width and depth that is suitable for placement between parallel joist, studs, rafters, and other parallel support structures that are uniformly spaced apart.
Foam insulation board is usually placed on the exterior sheathing surrounding the structure and is a good insulator against conductive and convective heat transfer. Foam insulation board is usually made up of extruded or pressed sheets made of expanded polystyrene (EPS), extended polystyrene (XPS), and polyisocyanurate (polyiso). Rigid foam board insulation provide high R-value with minimum thickness; low weight allows for easier installation and shipping cost; energy required to produce polystyrene foam insulation is 24 percent less than the energy required to make the equivalent R-value of fiberglass insulation (green product); High resistance to mold.
In the recent past, an additional sheet of reflective material has been applied to one of the broad surfaces of the fibrous blanket and foam insulation board. The radiant reflective surface can be effective in reflecting radiant heat if the reflective surface faces the source of the radiant heat and there is an approximately a ¾ inch air space between the reflective surface and another surface.
Radiant reflective insulation can be as simple as aluminum foil to multiple layered assemblies such as foil-bubble-foil with foam insulation possibly added as a layer. The reflective material, such as aluminum reflective insulation, can functions as a barrier to radiant heat transfer when applied in the right location and having the needed air space between the reflective surface and another material. Reflective insulation has many benefits: very thin; can be stabled and glued; easy to cut; does not promote mildew, mold, or fungus growth; reflective insulation can have, when applied correctly, a higher R-value than most other type of insulation. A ¼ inch of reflective insulation has more insulation than 6 inches of common fiberglass bat.
But most of the radiant reflective insulation on the market today does not address one of the major requirements for effective radiant reflection. One of the problems or inefficiencies of many of the radiant reflective applications is that when reflective surfaces of the foil engage another surface, such as the fiberglass of an adjacent insulation blanket or the adjacent gypsum foam board or other wall structure, such as siding, the foil looses a considerable amount of its ability to reflect heat (air space requirement). The reflective side of the radiant insulation needs to face the source of the radiant energy to effectively reflect back radiant heat. When radiant reflective material touches or is too close to a solid object much of the radiant heat is converted to conductive heat. Also the less material the reflected back radiant energy has to move through the better the total percentage amount that radiant heat is transferred out of the area being temperature controlled.
Another problem with the use of reflective surface on combination with other insulation materials is that the surface should become dirty from an accumulation of dust, trash, fibers, vapors, etc., the reflective sheet loses its ability to reflect radiant heat. There needs to be a protective covering spaced apart from the reflective surface.
One of the most significant factors that can limit the effective use of the current radiant reflective insulation is that they collapse or compress when roofing, wall siding or other hard material is applied resulting in inefficient radiant heat reflection. In most of the current radiant insulation manufactured assemblies the air space is usually not wide enough for effective radiant heat reflection and any compression on their surface further reduces the needed radiant reflective air space.